1. Field of the Invention
The present invention relates generally to the manufacture of circuit boards, and particularly to the removal and reattachment of integrated circuit package units to circuit boards.
2. Description of the Related Art
Contemporary electronic manufacture, including the packaging of integrated circuit modules and the like, has been filled with many new developments. The continued direction is one of a continual increase in circuitry density. As the integrated circuitry technology continues to advance, new means of attachment of the integrated circuit packages to the printed circuit board are developed. The prevailing new interconnect technology is surface mount technology. The increasing number of "Input/Outputs" into the integrated circuit units and the reduction in their size have placed even greater demands on manufacturers for reliability on this interconnect technology. The two primary IC packaging architectures in surface mount technology are (i) peripheral interconnecting devices wherein the I/P pins are arrayed around the periphery of the electronic package, (eg. Quad Flat Packs) and (ii) the array interconnecting pattern where the contacts are distributed over the underside of the entire substrate area, (eg. Ball Grid Array Packages.) The absence of the fine, fragile leads on the BGA package is believed to enable the achievement of higher reliability and less defect rate of such units on assembly, with the result that the BGA package is becoming the electronic assembly package of choice for critical, high I/O Integrated Circuits.
While BGA packages have reduced the complexity and concomitant likelihood for defects in manufacture, their electronic assembly still requires the ability and corresponding equipment necessary to perform rework (i.e. the removal and replacement of a defective component). A number of commercial BGA rework stations are presently available. The critical elements of commercial BGA rework stations are: closely controlled temperature profiles, both spatially and temporally, of both the circuit board substrate and the BGA package; and accurate alignment of the BGA package with respect to the attachment pads on the circuit board substrate.
After detachment and re-attachment of the BGA package has been accomplished, there is normally no way to know with certainty whether defects such as misalignment, bridging or missing balls are present until electrical test, particularly as the majority of the critical contact points are hidden from view. Under such circumstances, it would be desirable to employ a form of inspection that can penetrate the assembly, such as x-ray inspection.
It is known that X-ray inspection techniques are used for quality control in the production of electronic components, such as integrated circuits, multi-layer printed circuit boards and surface mounted components. Conventional X-ray inspection systems produce an X-ray image of the object being inspected. In addition, X-ray inspection systems are used for producing images of patients during medical treatments.
U.S. Pat. No. 4,995,068 describes a radiation therapy imager for providing X-ray images of a patient while being treated on a radiation therapy machine for the purpose of verification and monitoring of the position of the patient. A fluoroscopic plate produces a fluoroscopic image of the patient. The fluoroscopic plate has a detection resolution of 1.6 lp/mm. A first remote camera picks up the fluoroscopic image. An image of a simulator radiograph is picked up by a second remote camera. A remote console has the capability of receiving input from both the first and second cameras and for aligning the fluoroscopic image with the radiograph film image. The technology described in U.S. Pat. No. 4,995,068 has the shortcoming that the observer is viewing two different objects from two different locations. Also, the resolution of the images from a fluoroscopic plate is inadequate for the inspection of an electronic component such as a BGA package, where the majority of the electrical contacts are hidden from view.
U.S. Pat. Nos. 4,890,313 and 5,127,032 describe X-ray imaging systems for indicating alignment of X-ray beams with patient areas to be inspected. X-rays are projected at a 90.degree. angle to each other. A fluorescence screen is excited to emit light after receiving the X-rays. A thin deflection mirror is used to locate a video camera out of the path of the X-rays but in position to receive and record X-ray images on the fluorescence screen.
U.S. Pat. Nos. 4,974,249 and 5,113,425 issued to the inventor of this disclosure describe an X-ray inspection system for producing both film and fluoroscopic images of electronic components and assemblies therefor. An X-ray cabinet includes an X-ray tube and a slidable drawer for supporting the object to be inspected and film. An aperture is formed in the drawer to the X-ray tube. A fluoroscopic imaging device attaches to the slidable drawer for converting an X-ray image of the object to a fluoroscopic X-ray image. An output of the fluoroscopic imaging device is optically coupled to a video monitor. The fluoroscopic imaging device includes a thinly coated radioluminescent phosphor plate optically coupled to the input of an image intensifier.
In one embodiment, the image intensifier may include a microchannel plate to multiply the electrons before they are presented to the phosphor screen for reconversion to a light beam. Other intensifiers are also commercially available that do not include the microchannel plate, and may instead rely on, e.g. a high voltage supply and a demagnifying electronic lens system to provide a relatively high resolution image of the object. The present invention comprehends and extends without limitation to the use of all such fluoroscopic imaging devices within its scope.
Japanese Patent No. 54158984 describes an X-ray fluoroscopic inspection apparatus. X-rays are transmitted through a mirror. A camera picks up the transmitted X-rays after they pass through the object to be inspected. Visible rays from the object to be inspected reflect off the mirror and are picked up by an IVT camera. This patent has the drawback of scattering the X-rays with the mirror, thereby making the system unsuitable for detecting flaws in electronic components.
Conventional mirrors are formed of silica (glass) and oxygen. Silica has an atomic number of 14. When silica is used in the path of an X-ray beam, the silica absorbs the X-rays and re-emits secondary X-rays which effect is referred to in the prior art as scattering or Compton scattering. Scattering has the effect of formation of image noise which degrades the X-ray or fluoroscopic image. The degradation of the X-ray image is disadvantageous for high resolution inspection systems which congruently combine an optical image and an X-ray image of an object to be inspected.
Other patents were considered in the evaluation of the present invention. Accordingly. U.S. Pat. No. 5,654,994 to Marto relates to a manner for examining and detecting the motion of an injection valve in a fuel injection device. In this instance, and as is parent from the figure, x-ray radiation penetration means is disposed transverse to the axial direction of the valve, and lies outside the valve to facilitate inspection of motion. The Marto technique and device would not provide a view of the hidden contacts of a BGA package.
U.S. Pat. No. 5,372,294 to Gore et al. relates to a method for automated placement of components such as computer chips on a circuit board, where the component is initially placed on a transparent surface on which light is shined, so that the alignment of the components can be assured prior to assembly. However, the Gore et al. disclosure does not provide a means for inspecting the BGA product during and after its assembly.
U.S. Pat. No. 5,184,768 to Hall et al. instance relates to the use of a specific type of solder joint where a verification system is employed, to attempt to properly inspect such joints as may be hidden from normal visual inspection. The disclosure includes the use of x-ray means after the soldering joints have been formed, however is predicated on the use of a specific flow pattern and point of connection. Hall et al. however, does not provide the procedure or equipment for the accurate inspection of the electrical contacts of the BGA package, both during and after product assembly.
U.S. Pat. No. 5,012,502 to Battin et at. identify a specific technique for the determination of blind solder interconnections, where the specific region under inspection will differ in size from an adjacent region on a contiguous part, and therefore, the inspection of joint integrity as determined by solder flow will be more capable of measurement and inspection. Battin et al. do not disclose a technique or equipment that would facilitate the accurate real time inspection of BGA package assembly.
U.S. Pat. No. 4,944,447 to Thome relates to the use of x-ray inspection equipment in conjunction with a bonding verification process and in similar fashion to the previous cited references, inspects the products as part of the bonding process. The manner in which the x-ray inspection takes place is distinguishable, in that the products in question simply traverse a conveyor which passes through the x-ray inspection equipment. Thome et al. do not respond to the need for a means and related method for the real time inspection of BGA packaging during re-work.
U.S. Pat. No. Re. 35,423 to Adams et al. relate to the inspection of already completed and manufactured circuit boards by use of a real-time digital x-ray radiographic technique. The distinction between Adams et al. and the present invention lies in the fact that, among other things, the present invention engages an inspection as part of the manufacturing process, but more importantly, does so by the use of an apparatus which is characterized by several of its critical parts including the heating means, being prepared by x-ray transmissive materials. Such a construction and device are not shown in Adams et al., and therefore the patent disclosure thereof is distinguishable.
U.S. Pat. No. 5,524,132 to Ranadive relates to apparatus and process for reviewing manufacturing defects in test pieces such as rigid printed circuit boards, and again, is reflective of the post manufacturing inspection processes that are in broad and present use. The disclosure of this patent does not relate to the application of x-ray inspection as part of the manufacturing process, and therefore, does not respond to the need for thorough, real time inspection of BGA packages during assembly.
From the foregoing, it is apparent that a need exists for an efficient and reliable procedure and equipment for inspecting the construction and rework of BGA packages in real time, and it is toward the fulfillment of this need that the present invention is directed.